Hello, i wanted to put a cover on opening of 33

Hello,

i wanted to put a cover on opening of 33" x 23" on my unused fireplace as it has draft. I noticed the flue even is close has a gap so that might where the draft came from.

I am thinking to use this but i am unusre if its environmentally friendly?

https://www.lowes.ca/foam-board-insulation/dow-cladmate-4-ft-x-8-ft-extruded-polystyrene-insulation_10291799.htmlThanks

Should i just use a plywood and paint with anti-mold+mildew interior paint or use a pressure treated plywood?

I will not use this fireplace anymore.

Thanks
Clayton